<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style>
	*{
		padding:0;
		margin:0;
	}
	.box{
            width: 800px;
            height: 400px;
            border-radius: 200px;
            background-color: #ccc;
            margin:100px auto;
            position: relative;
        }
    .rec{
    	position: absolute;
    	width:200px;
    	height:200px;
    	background-color: red;
    	left:50%;
    	top:50%;
    	transform: translate(-50%,-50%);

    }
	</style>
</head>
<body>
	<div class="box">
    <div class="rec"></div>
</div>
</body>
</html>